Anti-Ia antisera have been shown capable of blocking binding of fluoresceinated Ig aggregates to lymphocytes. This blocking of Fc receptor activity was specific for anti-Ia antibody and was not due to anti-H-2 antibody. In view of these findings, it was deemed worthwhile to attempt isolation of Fc receptors to determine whether they were structurally associated with Ia antigen molecules or merely fortuitously close on the cell surface. Further attempts will be made to eliminate nonspecifically sticky material. A control of F(ab)2 aggregate- Sepharose will be introduced. Other possible artifacts such as sticking of labelled IgM will be examined.